This paper presents the design process and development approach for a method of maintaining sufficient quantities of water for research animals during a Shuttle mission of long duration. An inflight refill unit (IRU) consisting of two major subsystems, a fluid pumping unit (FPU) and a collapsible water reservoir (CWR), were developed. The FPU provides the system measurement and controls, pump, water lines, and plumbing necessary to collect water coming into the unit from the potable water system and pump it out into the RAHF drinking water tanks. The CWR is a Kevlar (TM) reinforced storage bladder connected to the FPU, which has a capacity of 6 liters in its expanded volume and functions to store the water collected from the potable water system, allowing for transport of the water back to the Spacelab where it is pumped into each of two research animal holding facilities. Additional components of the IRU system include the inlet and outlet fluid hoses, a power cable for providing 29V direct current spacecraft electrical power to the pump within the FPU, a tether system for the unit when in use in Spacelab, and an adapter for mating the unit to the orbiter waste collection system in order to dump excess water after use in Spacelab.